This invention relates to measuring the conductivity of a liquid and, more particularly, to an improved method and apparatus for continuously measuring the absolute conductivity of a production electrocoat paint wherein a bias voltage is imposed on electrodes of a conductivity measuring instrument to prevent fouling and repel the paint from the electrodes.
The conductivity of a solution such as paint is generally monitored using either one of two basic methods. The first method measures conductivity directly by maintaining a fixed voltage between two electrodes immersed in the solution so that the resulting current flow is directly proportional to the conductivity. In the second method, the electrodes may be supplied with a constant current flow so that the potential between them is directly proportional to the resistivity of the solution, which is the reciprocal of the conductivity.
It has been found in the industry that close control of production electrocoat paint conductivity is necessary for good paint coverage of a substrate, uniform coating thickness, and minimization of pinholes. High conductivity of the paint can cause excessive paint film thickness and low conductivity can cause poor "throwing power." Also, if the conductivity of the paint becomes extremely high, ultrafilters can plug, requiring excessive maintenance. Due to the nature of cathodic or anodic electrocoat processes, most sensors for instruments currently available become fouled with paint. Fouling occurs because the voltage levels of the electrodes allow electrocoat paint solids to electrically plate onto the surfaces of the electrodes.
The present invention is directed to an improvement to the electrical conductivity measuring instrument of U.S. Pat. No. 4,751,466 for preventing paint from plating on the electrodes of the probe. This patent discloses an instrument for measuring the electrical conductivity of a liquid passing through a nonconducting tubular probe by means of four electrodes which are inserted into the probe and metering circuitry associated with the probe. An AC voltage is imposed on the first electrode such that current is injected into the liquid flowing through the probe with the current passing to the third electrode, both the first and the third electrodes being of low impedance for passing current through the liquid. The voltage is then measured across the second and fourth electrodes with the fourth electrode being grounded to shield the probe from any ground currents such that the current due to conduction through the liquid flowing in the probe is correctly measured.
However, problems still exist in that electrodes have a tendency to eventually become coated with the paint or other liquid being measured. These coatings can introduce a substantial impedance across the interface between the electrode and the liquid solution. This affects the accuracy of the conductivity reading by indicating a much lower conductivity value than is accurate for the liquid bath. This coating or buildup on the electrodes also necessitates periodic interruptions to permit inspection and cleaning of the electrodes. Hence, it would be desirable to provide a system for measuring the electrical conductivity of a liquid wherein a coating does not buildup on electrodes of the system which would be particularly applicable to measuring the electrical conductivity of an electrocoat paint without buildup of paint solids on the electrodes.